Gallium nitride semiconductor structures with compositionally-graded transition layer

ABSTRACT

The invention provides semiconductor materials including a gallium nitride material layer formed on a silicon substrate and methods to form the semiconductor materials. The semiconductor materials include a transition layer formed between the silicon substrate and the gallium nitride material layer. The transition layer is compositionally-graded to lower stresses in the gallium nitride material layer which can result from differences in thermal expansion rates between the gallium nitride material and the substrate. The lowering of stresses in the gallium nitride material layer reduces the tendency of cracks to form. Thus, the invention enables the production of semiconductor materials including gallium nitride material layers having few or no cracks. The semiconductor materials may be used in a number of microelectronic and optical applications.

RELATED APPLICATIONS

This is a continuation of application Ser. No. 14/083,923 filed Nov. 19,2013.

This is a continuation of application Ser. No. 13/728,956 filed Dec. 27,2012.

This is a continuation of application Ser. No. 13/359,892 filed Jan. 27,2012.

This is a continuation of application Ser. No. 12/343,616 filed Dec. 24,2008.

This application is a divisional of U.S. patent application Ser. No.10/675,798, filed Sep. 30, 2003, and entitled “Gallium Nitride Materialsand Methods”, which is a continuation of U.S. patent application Ser.No. 09/736,972, filed Dec. 14, 2000, and entitled “Gallium NitrideMaterials and Methods”, both of which are incorporated herein byreference in their entireties.

FIELD OF INVENTION

The invention relates generally to semiconductor materials and, moreparticularly, to gallium nitride materials and methods of producinggallium nitride materials.

BACKGROUND OF INVENTION

Gallium nitride materials include gallium nitride (GaN) and its alloyssuch as aluminum gallium nitride (AlGaN), indium gallium nitride(InGaN), and aluminum indium gallium nitride (AlInGaN). These materialsare semiconductor compounds that have a relatively wide, direct bandgapwhich permits highly energetic electronic transitions to occur. Suchelectronic transitions can result in gallium nitride materials having anumber of attractive properties including the ability to efficientlyemit blue light, the ability to transmit signals at high frequency, andothers. Accordingly, gallium nitride materials are being widelyinvestigated in many microelectronic applications such as transistors,field emitters, and optoelectronic devices.

In many applications, gallium nitride materials are grown on asubstrate. However, differences in the properties between galliumnitride materials and substrates can lead to difficulties in growinglayers suitable for many applications. For example, gallium nitride(GaN) has a different thermal expansion coefficient (i.e., thermalexpansion rate) than many substrate materials including sapphire,silicon carbide, and silicon. This difference in thermal expansion canlead to cracking of a gallium nitride layer deposited on such substrateswhen the structure is cooled, for example, during processing. Thecracking phenomena can prevent gallium nitride materials from beingsuitable for use in many applications. Cracking can be particularlyproblematic for relatively thick (e.g., >0.5 micron) gallium nitridelayers.

Gallium nitride (GaN) also has a different lattice constant than mostsubstrate materials. The difference in lattice constant may lead to theformation of defects in gallium nitride material layers deposited onsubstrates. Such defects can impair the performance of devices formedusing the gallium nitride material layers.

Prior art techniques have been developed to address crack formation anddefect formation in gallium nitride materials deposited on sapphiresubstrates and silicon carbide substrates. Such techniques, for example,may involve depositing one or more buffer layers on the substrate and,then, depositing the gallium nitride material on the buffer layer(s).

SUMMARY OF INVENTION

The invention provides semiconductor materials including a galliumnitride material layer formed on a silicon substrate and methods to formthe semiconductor materials. The semiconductor materials include atransition layer formed between the silicon substrate and the galliumnitride material layer. The transition layer is compositionally-gradedto lower stresses in the gallium nitride material layer which can resultfrom differences in thermal expansion rates between the gallium nitridematerial and the substrate. The lowering of stresses in the galliumnitride material layer reduces the tendency of cracks to form whichenables the production of semiconductor materials including galliumnitride material layers having few or no cracks. The semiconductormaterials may be used in a number of microelectronic and opticalapplications.

The invention has overcome the problem of growing gallium nitridematerials having few or no cracks on silicon substrates which, due tothe large differences in both thermal expansion rate and latticeconstant between gallium nitride materials (e.g., GaN) and silicon, isconsiderably more challenging than on other types of substrates (e.g.,SiC and sapphire).

In one aspect, the invention provides a semiconductor material. Thematerial includes a silicon substrate and a compositionally-gradedtransition layer formed over the silicon substrate. The material furtherincludes a gallium nitride material layer formed over the transitionlayer.

In another aspect, the invention provides a semiconductor material. Thesemiconductor material includes a silicon substrate and a galliumnitride material layer formed over the silicon substrate. The galliumnitride material layer has a crack level of less than 0.005 μm/μm².

In another aspect, the invention provides a semiconductor structure. Thesemiconductor structure includes a silicon substrate, and a galliumnitride material layer formed over the silicon substrate. The galliumnitride material layer has a thickness of greater than 0.5 micron. Thesemiconductor structure forms a semiconductor device.

In another aspect, the invention provides a method of producing asemiconductor material. The method includes the steps of forming acompositionally-graded transition layer over a silicon substrate, andforming a gallium nitride material layer over the transition layer.

In another aspect, the invention provides a method of producing asemiconductor material. The method includes forming a gallium nitridematerial layer formed over a silicon substrate. The gallium nitridematerial layer has a crack level of less than 0.005 μm/μm²

In another aspect, the invention provides a method of forming asemiconductor structure. The method includes forming a semiconductorstructure comprising a silicon substrate, and a gallium nitride materiallayer formed over the silicon substrate. The gallium nitride materiallayer has a thickness of greater than 0.5 micron.

In another aspect, the invention provides a semiconductor material. Thesemiconductor material comprises a silicon (100) substrate and a galliumnitride material layer having a Wurtzite structure formed over thesilicon substrate.

Other advantages, aspects, and features of the invention will becomeapparent from the following detailed description of the invention whenconsidered in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a semiconductor material including acompositionally-graded transition layer according to one embodiment ofthe present invention.

FIGS. 2A to 2I are exemplary profiles of the concentration of an elementas a function of the thickness across the compositionally-gradedtransition layer.

FIGS. 3A and 3B illustrate a semiconductor material that includes asuperlattice transition layer according to another embodiment of thepresent invention.

FIGS. 4A and 4B are exemplary profiles of the concentration of anelement as a function of the thickness of the transition layers in thesemiconductor materials of FIGS. 3A and 3B, respectively.

FIG. 5 illustrates a textured substrate used to form the semiconductormaterial according to one embodiment of the present invention.

FIG. 6 illustrates a semiconductor material including an intermediatelayer between the substrate and the transition layer according toanother embodiment of the present invention.

FIG. 7 illustrates an LED formed from the semiconductor materialaccording to another embodiment of the present invention.

FIG. 8 illustrates a laser diode formed from the semiconductor materialaccording to another embodiment of the present invention.

FIG. 9 illustrates a FET formed from the semiconductor materialaccording to another embodiment of the present invention.

FIG. 10 is a micrograph of the surface of a gallium nitride layer formedon a silicon substrate with a transition layer as described in Example1.

FIG. 11 is a micrograph of the surface of a gallium nitride layer formedon a silicon substrate without a transition layer as described incomparative Example 2.

DETAILED DESCRIPTION OF THE INVENTION

The invention provides semiconductor materials including a galliumnitride material layer and a process to produce the semiconductormaterials. As used herein, the phrase “gallium nitride material” refersto gallium nitride and any of its alloys, such as aluminum galliumnitride (Al_(x)Ga_((1-x))N), indium gallium nitride (In_(y)Ga_((1-y))N),aluminum indium gallium nitride (Al_(x)In_(y)Ga_((1-x-y))N), galliumarsenide phosporide nitride (GaAs_(a)P_(b) N_((1-a-b))), aluminum indiumgallium arsenide phosporide nitride (Al_(x)In_(y)Ga_((1-x-y))As_(a)P_(b)N_((1-a-b))), amongst others. Typically, when present, arsenic and/orphosphorous are at low concentrations (i.e., less than 5 weightpercent).

Referring to FIG. 1, a semiconductor material 10 according to oneembodiment of the invention is shown. Semiconductor material 10 includesa transition layer 12 formed over a silicon substrate 14 and a galliumnitride material layer 16 formed over the transition layer. As describedfurther below, transition layer 12 is compositionally-graded to reduceinternal stresses within gallium nitride material layer 16 that canresult from differences between the thermal expansion rates of substrate14 and the gallium nitride material layer. The internal stresses mayarise, for example, when semiconductor material 10 is cooled after thedeposition of gallium nitride material layer 16 and substrate 14contracts more rapidly than the gallium nitride material layer. As aresult of the reduced internal stresses, gallium nitride material layer16 can be formed with a low crack level making semiconductor material 10suitable for use in a number of applications including FETs, LEDs, laserdiodes, and the like.

It should be understood that when a layer is referred to as being “on”or “over” another layer or substrate, it can be directly on the layer orsubstrate, or an intervening layer may also be present. It should alsobe understood that when a layer is referred to as being “on” or “over”another layer or substrate, it may cover the entire layer or substrate,or a portion of the layer or substrate.

As used herein, the term “compositionally-graded layer” refers to alayer having a composition that varies across at least a portion of thethickness of the layer. Thus, transition layer 12 includes at least twodifferent compositions at different depths within the layer. Asdescribed further below, the composition of transition layer 12 can bevaried in a number of ways. It is generally advantageous to vary thecomposition of transition layer 12 in a manner that provides sufficientstrain relief to limit or prevent the formation of cracks in galliumnitride material layer 16.

According to one set of embodiments, transition layer 12 is composed ofan alloy of gallium nitride such as Al_(x)In_(y)Ga_((1-x-y))N,Al_(x)Ga_((1-x))N, and In_(y)Ga_((1-y))N. It should be understood,however, that transition layers having other compositions may also beused. In embodiments which utilize alloys of gallium nitride, theconcentration of at least one of the elements (e.g., Ga, Al, In) of thealloy is typically varied across at least a portion of the thickness ofthe transition layer. When transition layer 12 has anAl_(x)In_(y)Ga_((1-x-y))N composition, x and/or y is varied. Whentransition layer 12 has a Al_(x)Ga_((1-x))N composition, x is varied.When transition layer 12 has a In_(y)Ga_((1-y))N composition, y isvaried.

In certain preferred embodiments, it is desirable for transition layer12 to have a low gallium concentration at back surface 18 and a highgallium concentration at front surface 20. It has been found that suchtransition layers are particularly effective in relieving internalstresses within gallium nitride material layer 16. Decreasing thegallium concentration of a gallium nitride alloy transition layer canmake the thermal expansion rate of the alloy more similar to the thermalexpansion rate of silicon. As described further below, gallium nitridematerial layer 16 typically includes a high gallium concentration. Thus,in these embodiments, increasing the concentration of gallium intransition layer 12 can make the thermal expansion rate of the alloymore similar to the thermal expansion rate of gallium nitride materiallayer 16. It is believed that in these preferred embodiments effectivestrain relief is achievable because back surface 18 has a relativelysimilar thermal expansion rate as substrate 14, while front surface 20has a relatively similar thermal expansion rate as gallium nitridematerial layer 16.

In some cases, the sum of (x+y) at back surface 18 is greater than 0.4,greater than 0.6, greater than 0.8 or ever higher. In some preferredembodiments, (x+y)=1 at back surface 18, so that transition layer 12 isfree of gallium at the back surface. In some cases, the sum of (x+y) isless than 0.3, less than 0.2, or even less at front surface 20. In somepreferred embodiments, the sum of (x+y)=0 at front surface 20, so thattransition layer 12 has a composition of GaN at the front surface. Itmay be particularly preferred for transition layer 12 to have acomposition of GaN at front surface 20, when gallium nitride materiallayer 16 has a composition of GaN. In other cases when gallium nitridematerial layer is composed of an alloy of GaN, it may be preferable forthe composition of transition layer 12 at front surface 20 to be thesame as the composition of gallium nitride material layer 16. In somecases, transition layer 12 is free of gallium at back surface 18 and hasa composition of GaN at front surface 20.

In certain embodiments, it may be preferable for transition layer 12 tocomprise Al_(x)Ga_((1-x))N. In these cases, the transition layer is freeof indium. In other cases, transition layer 12 may include a smallamount of indium, for example, less than 10 percent by weight. Whenindium is present in transition layer 12 (i.e.,Al_(x)In_(y)Ga_((1-x-y))N), the concentration of indium (i.e., y) mayremain constant throughout the transition layer, while the concentrationof gallium and aluminum are graded.

The composition in transition layer 12 may be graded across itsthickness in a number of different manners. For example, the compositionmay be graded continuously, discontinuously, across the entirethickness, or across only a portion of the thickness. As describedabove, the composition may be graded by varying the concentration of oneor more of the elements (i.e., Ga, Al, In). FIGS. 2A to 2I illustrateexemplary manners in which the composition may be graded by varying theconcentration of one of the elements as a function of thickness acrosstransition layer 12. In certain preferred embodiments, the profilesrepresent the concentration of gallium across the thickness oftransition layer 12, though it should be understood that in otherembodiments the profiles may represent the concentration of otherelements (i.e., Al or In). The convention in FIGS. 2A to 2I is that thethickness of transition layer 12 increases in the direction away fromsubstrate 14 (i.e., t=0 at back surface 18 and t=1 at front surface 20).

FIG. 2A shows a step-wise variation of concentration as a function ofthickness which includes multiple steps. FIG. 2B shows a step-wisevariation of concentration as a function of thickness which includes twosteps. FIG. 2C shows a saw tooth variation of concentration as afunction of thickness. FIG. 2D shows continuous variations ofconcentration at a constant rate as a function of thickness. FIG. 2Eshows a continuous variation of concentration at a constant ratestarting from a non-zero concentration. FIGS. 2F and 2G show continuousvariations of concentration as a function of thickness at exponentialrates. FIG. 2H shows a discontinuous variation of concentration as afunction of thickness. FIG. 2I shows a variation of the concentrationacross a portion of the thickness of the transition layer.

It should be understood that the profiles illustrated in FIGS. 2A to 2Iare intended to be exemplary and that the composition of transitionlayer 12 may be graded in other manners that are within the scope of thepresent invention.

Referring to FIGS. 3A and 3B, transition layer 12 may be acompositionally-graded strained layer superlattice 22 according toanother embodiment of the present invention. Superlattice 22 includesalternating layers 24 a, 24 b of semiconductor compounds havingdifferent compositions. In some cases, the composition across eachindividual layer 24 a, 24 b is varied according to any of the mannersdescribed above. In other cases, the composition of individual layers 24a, 24 b is constant across the thickness of the individual layer asshown in the concentration profile of FIGS. 4A and 4B. As shown in FIGS.3B (and 4B), the thicknesses of individual layers 24 a, 24 b is variedacross transition layer 12 to provide compositional grading.

In one preferred set of embodiments, superlattice 22 comprisesalternating layers of gallium nitride alloys having differentcompositions. For example, layer 24 a has a composition ofAl_(x)In_(y)Ga_((1-x-y))N and layer 24 b has a composition ofAl_(a)In_(b)Ga_((1-a-b))N, wherein x≠a and y≠b. In cases when thecomposition is graded across each individual layer, the concentration ofat least one of the elements (i.e., Al, In, Ga) of the alloy can bevaried according to any of the manners described above. In cases whenthe composition of individual layers 24 a, 24 b is constant (FIG. 3B),the gallium concentration is typically different in individual layers 24a, 24 b.

As described above, it may be desirable to have a low galliumconcentration at back surface 18 and a high gallium concentration atfront surface 20. In embodiments that utilize superlattice 22 astransition layer 12, increasing the gallium concentration in a directionaway from back surface 18 can be accomplished by varying the thicknessof individual layers. As shown in FIGS. 3B and 4B, layer 24 a has a lowgallium concentration and layer 24 b has a high gallium concentration.As shown, layers 24 a are relatively thick and layers 24 b arerelatively thin proximate back surface 18. The thickness of layers 24 ais decreased and the thickness of layers 24 b is increased in adirection away from back surface 18. Thus, layers 24 a are relativelythin and layers 24 b are relatively thick proximate front surface 20.This structure provides a low gallium concentration at back surface 18and a high gallium concentration at front surface 20.

It should be understood that transition layer 12 may be formed of acombination of a single layer having a graded composition and asuperlattice. In some cases, the superlattice is formed over the singlecompositionally-graded layer. In other cases, the singlecompositionally-graded layer is formed over the superlattice.

Transition layer 12 can have a variety of thicknesses depending on theapplication. Generally, though not always, transition layer 12 has athickness of less than about 500 microns. In some cases, relativelythick transition layers are preferable, for example between about 2.0microns and about 20 microns. Thick transition layers may be preferredwhen thick gallium nitride material layers (i.e., greater than 5microns) are produced. In some cases, relatively thin transition layersare preferable, for example between about 0.03 micron and about 2.0microns. When superlattice structures are used as transition layers, thethickness of individual layers 24 a, 24 b depends upon the particularapplication. Typically the thickness of individual layers 24 a, 24 b maybe between about 0.001 microns and about 0.020 microns. As describedabove, the thicknesses of individual layers may vary across transitionlayer 12 (FIG. 3B).

Gallium nitride material layer 16 is formed of gallium nitride (GaN) orany of its alloys including aluminum gallium nitride(Al_(x)Ga_((1-x))N), indium gallium nitride (In_(y)Ga_((1-y))N), andaluminum indium gallium nitride (Al_(x)In_(y)Ga_((1-x-y))N). Thecomposition of gallium nitride material layer 16 is generally constantacross its thickness as distinguished with transition layer 12. Thus, xand/or y are generally fixed when gallium nitride material is formed ofany of the aforementioned compound alloys. It should be understood thatsmall variations in the composition of gallium nitride material layer 16may occur, for example, as a result of slight non-uniformities andinhomogeneities during growth.

In certain preferred embodiments, gallium nitride material layer 16 hasa high concentration of gallium and includes little or no amounts ofaluminum and/or indium. In high gallium concentration embodiments, thesum of (x+y) may be less than 0.4, less than 0.2, less than 0.1, or evenless. In some cases, it is preferable for gallium nitride material layer16 to have a composition of GaN (i.e., x+y=0).

As described above, gallium nitride material layer 16 has a low cracklevel as a result of the ability of transition layer 12 to relievestress arising from differences in thermal expansion rates between thesilicon substrate and the gallium nitride material. A “crack,” as usedherein, is a linear fracture or a cleavage having a length to widthratio of greater than 5:1 that extends to the surface of the galliumnitride material. It should be understood that a crack may or may notextend through the entire thickness of the gallium nitride material.“Crack level” is defined as a total measure of all crack lengths in agallium nitride material per unit surface area. Crack level can beexpressed in units of μm/μm². The crack level of a gallium nitridematerial can be measured, for example, using optical microscopytechniques. To determine the crack level, the length of all of thecracks in a given area (i.e., 1 mm×1 mm) are added together and dividedby the total surface area. If necessary, this process may be repeated ata number of locations across the surface to provide a measurementrepresentative of the entire gallium nitride material. The crack levelat each location may be averaged to provide a crack level for thematerial. The number of locations depends upon the amount of surfacearea of the gallium nitride material. When measuring the crack level ofa gallium nitride material, measurements are not made within a regionproximate to edges of the material known as an edge exclusion. Thenominal edge exclusion is 5 mm from the edge. Edge effects in suchregions may lead to increased crack levels and are typically not used toin device formation.

Gallium nitride material layer 16 advantageously has a low crack level.In some cases, gallium nitride material layer 16 has a crack level ofless than 0.005 μm/μm². In some cases, gallium nitride material has avery low crack level of less than 0.001 μm/μm². In certain cases, it maybe preferable for gallium nitride material layer 16 to be substantiallycrack-free as defined by a crack level of less than 0.0001 μm/μm².

Gallium nitride material layer 16 preferably has a monocrystallinestructure. In preferred cases, gallium nitride material layer 16 has aWurtzite (hexagonal) structure. Preferably, the entire gallium nitridematerial layer has a Wurtzite structure. The gallium nitride materiallayer 16 is generally of high enough quality so as to permit theformation of devices therein. In some embodiments, gallium nitridematerial layer 16 has a relatively low amount of defects (e.g., lessthan 10⁹ cm⁻²) which, for example, result from the lattice mismatchbetween gallium nitride and silicon.

The thickness of gallium nitride material layer 16 is dictated, in part,by the requirements of the specific application. In applications whengallium nitride material is used as a device layer, the thickness issufficient to permit formation of the device. Gallium nitride materiallayer 16 generally has a thickness of greater than 0.1 micron, thoughnot always. In other cases, thicker gallium nitride material layers aredesired such as thicknesses greater than 0.5 micron, greater than 0.75micron, greater than 1.0 microns, greater than 2.0 microns, or evengreater than 5.0 microns. Even thick gallium nitride material layers 16are achievable at low crack densities because of the presence oftransition layer 12. In relatively thick gallium nitride layers, upperregions of the layer may include low amounts of defects due to thetendency of defects to annihilate one another as they propagatevertically through the layer. Thus, in these cases, the use of thickgallium nitride layers may improve device performance.

Silicon substrate 14 typically is formed of high-quality single-crystalsilicon as readily available in the art. Silicon substrates 14 havingdifferent crystallographic orientations may be used. In some cases,silicon (111) substrates are preferred. In other cases, silicon (100)substrates are preferred. Gallium nitride material layer 16 having aWurtzite structure may be grown on silicon (111) substrates and silicon(100) substrates using transition layer 12. It is particularlysurprising that gallium nitride material layer 16 having a Wurtzitestructure may grown on silicon (100) substrates because conventionaltechniques generally result in gallium nitride materials having amixture of zinc blend (cubic) and Wurtzite structures when grown onsilicon (100) substrates.

Silicon substrate may have any dimensions as used in the art. Suitablediameters include, but are not limited to, 2 inches, 4 inches, 6 inches,and 8 inches. In some embodiments, silicon substrate 14 is relativelythick, for example, greater than 250 microns. Thicker substrates aregenerally able to resist bending which can occur, in some cases, inthinner substrates.

As used herein, silicon substrate 14 refers to any substrate thatincludes a silicon layer at its top surface. Examples of suitablesilicon substrates include substrates that are composed entirely ofsilicon (e.g., silicon wafers), silicon-on-insulator (SOD substrates,silicon-on-sapphire substrate (SOS), SIMOX substrates, amongst others.

Referring to FIG. 5, silicon substrate 14 is textured according to someembodiments of the present invention. As illustrated, textured substrate14 includes a plurality of posts 24 which define trenches 26therebetween. Such texturing can be provided using selective etchingand/or selective epitaxial growth. Etching may be performed usingstandard dry or wet etching techniques, such as with a mask which latermay be removed. In some cases, textured substrates are used inconjunction with the transition layers described herein to grow galliumnitride material layers with very low defect densities (e.g., less than10⁷ cm⁻²). Silicon substrate 14 may also be pre-patterned to includemask areas which selectively expose regions of the substrate, whilecovering other regions. Such pre-patterned substrates enable selectivearea epitaxial growth which may be advantageous in minimizing defectdensities.

Referring to FIG. 6, semiconductor material 10 includes an intermediatelayer 28 between silicon substrate 14 and transition layer 12 accordingto another embodiment of the present invention. It should be understoodthat intermediate layer 28 may also be positioned between transitionlayer 12 and gallium nitride material layer 16. Intermediate layer 28,when provided, may further relieve stress in gallium nitride materiallayer 16. Intermediate layer generally has a thickness of less thanabout 500 microns and, in some cases, between about 0.01 micron andabout 2.0 microns. The presence of intermediate layer 28 may permitreduction of the thickness of transition layer 12. The composition ofthe intermediate layer is generally constant throughout its thickness.

Intermediate layer 28, for example, can be composed of a GaN alloy suchas aluminum gallium nitride (Al_(x)Ga_((1-x))N), indium gallium nitride(In_(y)Ga_((1-y))N), and aluminum indium gallium nitride(Al_(x)In_(y)Ga_((1-x-y))N). In these cases, the sum of (x+y) in theintermediate layer may be greater than 0.4, greater than 0.6, greaterthan 0.8, greater than 0.9, or even more. In some preferred cases, theintermediate layer is free of gallium and is composed of Al_(x)In_(y)Nor AlN. GaN alloy intermediate layers with low Ga concentrations may beeffective at relieving stresses because they have a thermal expansionrate relatively close to the thermal expansion rate of silicon substrate14.

It should be understood that intermediate layer 28 may be utilized inaccordance with any of the embodiments described herein includingembodiments that use a superlattice as a transition layer. Inembodiments in which transition layer 12 includes a singlecompositionally-graded layer and a superlattice, the intermediate layermay be positioned between the compositionally-graded layer and thesuperlattice. In some embodiments of the invention, more than oneintermediate layer 28 having different compositions may be provided.

According to one preferred method, transition layer 12 and galliumnitride material layer 16 are grown using a metalorganic chemical vapordeposition (MOCVD) process. It should be understood that other suitabletechniques known in the art may also be utilized to deposit transitionlayer 12 and gallium nitride material layer 16 including molecular beamepitaxy (MBE), hydride vapor phase epitaxy (HVPE), and the like.

Generally, the MOCVD process involves introducing different source gasesinto an environment (e.g., a process system) around a substrate andproviding conditions which promote a reaction between the gases to forma layer on the substrate surface. The reaction proceeds until a layer ofdesired thickness is achieved. The composition of the layer may becontrolled, as described further below, by several factors including gascomposition, gas concentration, and the reaction conditions (e.g.temperature and pressure).

Examples of suitable source gases for MOCVD growth of the transitionlayer include trimethylaluminum (TMA) or triethylaluminum (TEA) assources of aluminum; trimethylindium (TMI) or triethylindium (TEI) assources of indium; trimethylgallium (TMG) or trimethylgallium (TEG) assources of gallium; and ammonia (NH₃) as a source of nitrogen. Theparticular source gas used depends upon the desired composition of thetransition layer. For example, an aluminum source (e.g., TMA or TEA), agallium source (TMG or TEG), and a nitrogen source are used to depositfilms having an Al_(x)Ga_(1-x)N composition.

The flow rates of the source gases, the ratios of the source gases, andthe absolute concentrations of the source gases may be controlled toprovide transition layers having the desired composition. For the growthof Al_(x)Ga_(1-x)N layers, typical TMA flow rates are between about 5μmol/min and about 50 μmol/min with a flow rate of about 20 μmol/minbeing preferred in some cases; typical TMG flow rates are between about5 μmol/min and 250 μmol/min, with a flow rate of 115 μmol/min beingpreferred in some cases; and the flow rate of ammonia is typicallybetween about 3 slpm to about 10 slpm. The reaction temperatures aregenerally between about 900° C. and about 1200° C. and the processpressures are between about 1 Torr and about 760 Torr. It is to beunderstood that the process conditions, and in particular the flow rate,are highly dependent on the process system configuration. Typically,smaller throughput systems require less flow than larger throughputsystems.

Process parameters are suitably adjusted to control the compositionalgrading of the transition layer. The composition may be graded bychanging the process conditions to favor the growth of particularcompositions. For example, to increase incorporation of gallium in thetransition layer thereby increasing the gallium concentration, the flowrate and/or the concentration of the gallium source (e.g., TMG or TEG)may be increased. Similarly, to increase incorporation of aluminum intothe transition layer thereby increasing the aluminum concentration, theflow rate and/or the concentration of the aluminum source (e.g., TMA orTEA) may be increased. The manner in which the flow rate and/or theconcentration of the source is increased (or decreased) controls themanner in which the composition is graded. In other embodiments, thetemperature and/or pressure is adjusted to favor the growth of aparticular compound. Growth temperatures and pressures favoring theincorporation of gallium into the transition layer differ from thegrowth temperatures and pressures favoring the incorporation of aluminuminto the transition layer. Thus, the composition may be graded bysuitably adjusting temperature and pressure.

Typical growth rates of the transition layer are between about 0.01μm/hr and about 3.0 μm/hr. The growth rate depends upon the processparameters as well as the composition of the layer.

The gallium nitride material layer (and intermediate layers, if present)may also be grown using an MOCVD process. The process may utilize sourcegases and process parameters similar to those described above for thedeposition of the transition layer. The particular source gases andprocess parameters are selected based upon the desired composition. Whendepositing the gallium nitride material layer (or the intermediatelayer), however, the process parameters are maintained constant so as toprovide a film having a constant composition.

The semiconductor materials of the invention may be used in a variety ofapplications. In some cases, semiconductor material 10 is processedusing known techniques to form a semiconductor device. Doped regions maybe formed within gallium nitride material layer 16 and additional layersmay be deposited upon the gallium nitride material layer to produce thedesired semiconductor structure. In some embodiments, gallium nitridematerial layer 16 is doped using known techniques to achieve a desiredconductivity.

Any suitable semiconductor device known in the art including electronicand optical devices can be produced using semiconductor material 10.Exemplary devices include LEDs, laser diodes, FETs (e.g., HFETs) amongstothers.

FIG. 7 schematically illustrates an exemplary LED 30 formed fromsemiconductor material 10. LED 30 includes silicon-doped gallium nitridematerial layer 16 formed on transition layer 12 on silicon substrate 14.In the illustrative embodiment, the following layers are formed ongallium nitride material layer 16 in succession: a silicon-dopedAl_(x)Ga_((1-x))N layer 31 (containing 0-20% by weight Al), a GaN/InGaNsingle or multiple quantum well 32, a magnesium-doped Al_(x)Ga_((1-x))Nlayer 34 (containing 10-20% by weight Al), and a magnesium-doped GaNlayer 36. LED 30 includes a p-type metal contact 38 on magnesium-dopedGaN layer 36 and an n-type metal contact pad 39 on silicon-doped galliumnitride material layer 16. LED 30 may be provided as a variety ofdifferent structure including: a double heterostructure (e.g., Al>0% inlayer 31), a single heterostructure (e.g., Al=0% in layer 31), asymmetric structure, or an asymmetric structure. It should be understoodthat LED may have a variety of different structures as known to those ofordinary skill in the art.

FIG. 8 schematically illustrates an exemplary laser diode 40 formed fromsemiconductor material 10. Laser diode 40 includes silicon-doped galliumnitride material layer 16 formed on transition layer 12 on siliconsubstrate 14. In the illustrative embodiment, the following layers areformed on gallium nitride material layer 16 in succession: asilicon-doped Al_(x)Ga_((1-x))N layer 42 (containing 5-30% by weightAl), a silicon-doped Al_(x)Ga_((1-x))N layer 44 (containing 0-20% byweight Al), a GaN/InGaN single or multiple quantum well 46, amagnesium-doped Al_(x)Ga_((1-x))N layer 48 (containing 5-20% by weightAl), a magnesium-doped Al_(x)Ga_((1-x))N layer 50 (containing 5-30% byweight Al), and a magnesium-doped GaN layer 52. Laser diode 40 includesa p-type metal contact 38 on magnesium-doped GaN layer 52 and an n-typemetal contact pad 39 on silicon-doped gallium nitride material layer 16.It should be understood that laser diode 40 may have a variety ofdifferent structures as known to those of ordinary skill in the art.

FIG. 9 schematically illustrates a FET 54 (e.g., HFET) formed fromsemiconductor material 10. FET 54 includes intrinsic gallium nitridematerial layer 16 formed on transition layer 12 on silicon substrate 14.FEE 54 includes an Al_(x)Ga_((1-x))N layer 56 (containing 10-40% byweight Al). It should be understood that FET 54 may have a variety ofdifferent structures as known to those of ordinary skill in the art.

The function and advantage of these and other embodiments of the presentinvention will be more fully understood from the examples below. Thefollowing examples are intended to illustrate the benefits of thepresent invention, but do not exemplify the full scope of the invention.

Example 1 Production of Gallium Nitride Layer Using a CompositionallyGraded Transition Layer

This example illustrates the effectiveness of a compositionally-gradedtransition layer in limiting the number of cracks in a gallium nitridematerial grown on a silicon substrate.

An MOCVD process was used to grow an AlN intermediate layer, anAl_(x)Ga_(1-x)N compositionally-graded transition layer, and a GaN layerin succession on a silicon substrate.

A silicon substrate having a 2-inch diameter and a thickness of 250microns was positioned in an MOCVD system. To grow the AlN intermediatelayer, trimethylaluminum gas (TMA) was introduced into the MOCVD systemat a flow rate of about 50 μmol/min and ammonia gas (NH₃) was introducedinto the system at a flow rate of between about 3 and about 10 slpm. Agrowth temperature of between about 1000-1100° C. and a growth pressure30-200 Torr were maintained in the system. After about 60 minutes, anAlN intermediate layer was formed having a thickness of about 0.3 micronon the silicon substrate.

After the growth of the intermediate layer, trimethylgallium (TMG) wasintroduced into the system at a flow rate of about 5 μmol/min to providea ratio of TMA:TMG of about 10:1. To form the compositionally-gradedtransition layer, the flow rate of the TMA was decreased to about 5μmol/min, while the flow rate of TMG was increased to about 115μmol/min. Over this time, the ratio of TMA:TMG was decreased from about10:1 to about 1:23. After about 30 minutes, a compositionally-gradedtransition layer having a thickness of about 0.4 micron was grown on theintermediate layer.

To grow the gallium nitride layer on the transition layer, theintroduction of TMA into the system was stopped and the TMG flow ratewas adjusted to about 115 μmol/min. The flow rate of ammonia wasmaintained between about 3 and about 10 slpm. The growth temperature wasmaintained between about 1000 and about 1050° C. and the growth pressurebetween about 30 and about 200 Torr. After about 45 minutes, a GaN layerhaving a thickness of about 1.5 micron was grown on thecompositionally-graded transition layer. The semiconductor material wasfurnace-cooled to room temperature and removed from the MOCVD system foranalysis.

The resulting semiconductor material included a 0.3 micron MNintermediate layer formed on the silicon substrate; a 0.4 micron thickcontinuously graded Al_(x)Ga_(1-x)N transition layer formed on theintermediate layer; and a 1.5 micron GaN layer grown on the transitionlayer. The composition of the Al_(x)Ga_(1-x)N transition layer wasgraded from x=0.8 at the juncture with the intermediate layer to x=0 atthe juncture with the GaN layer. The GaN layer had a monocrystallinestructure.

The crack level of the semiconductor material was measured using anoptical microscopic technique. The microscope was equipped with thecamera capable of taking micrographs of the surface of the GaN layer.FIG. 10 is a micrograph showing a representative area of slightlygreater than 1 mm² on the surface of the GaN layer. No cracks arevisible in the representative area. Measurements were repeated atseveral other locations on the surface of the GaN layer and similarresults were achieved. The gallium nitride material was found to besubstantially crack-free as defined by a crack level of less than 0.0001μm/μm².

This example illustrates the ability to grow gallium nitride layershaving a low crack level on a silicon substrate using acompositionally-graded transition layer.

Comparative Example 2 Production of Gallium Nitride Layer without Usinga Compositionally Graded Transition Layer

This example illustrates the generation of cracks in a gallium nitridematerial grown on a silicon substrate without using acompositionally-graded transition layer.

An MOCVD process was used to grow an AlN intermediate layer and a GaNlayer in succession on a silicon substrate.

A silicon (111) substrate having a 2-inch diameter and a thickness of250 microns was positioned in the same MOCVD system as used inExample 1. An AlN intermediate layer was formed using essentially thesame processing conditions as the growth of the intermediate layer inExample 1. A GaN layer was grown on the intermediate layer usingessentially the same processing conditions as the growth of the GaNlayer in Example 1. A compositionally-graded transition layer was notgrown. The semiconductor material was furnace-cooled to room temperatureand removed from the MOCVD system for analysis.

The resulting semiconductor material included a 0.3 micron AlNintermediate layer formed on the silicon substrate, and a 1.5 micron GaNlayer grown on the intermediate layer. The GaN layer had amonocrystalline structure.

The crack level of the GaN layer was measured using the same techniqueas described in Example 1. FIG. 11 is a micrograph showing arepresentative area of slightly greater than 1 mm² on the surface of theGaN layer. The length of each crack in the area was measured and addedtogether to determine the total crack length. The total crack length wasdivided by the surface area to determine the crack level. Measurementswere repeated at several other locations on the surface which wereaveraged to provide a crack level of the GaN layer of about 0.007μm/μm².

This comparative example illustrates the presence of cracks in galliumnitride layers grown on a silicon substrate without using acompositionally-graded transition layer.

Those skilled in the art would readily appreciate that all parameterslisted herein are meant to be exemplary and that the actual parameterswould depend upon the specific application for which the semiconductormaterials and methods of the invention are used. It is, therefore, to beunderstood that the foregoing embodiments are presented by way ofexample only and that, within the scope of the appended claims andequivalents thereto the invention may be practiced otherwise than asspecifically described.

What is claimed is:
 1. A semiconductor structure comprising: atransition layer comprising an alloy of gallium nitride consisting ofAl_(x)In_(y)Ga_((1-x-y))N; a gallium nitride layer over said transitionlayer; wherein said transition layer is discontinuously graded.
 2. Thesemiconductor structure of claim 1, wherein a value of y of saidtransition layer is selected from the group consisting of zero andsubstantially zero.
 3. The semiconductor structure of claim 1, whereinsaid discontinuously graded transition layer is step-wise graded.
 4. Thesemiconductor structure of claim 1, wherein a value of 1-x-y at a frontsurface of said transition layer is greater than a value of 1-x-y at aback surface of said transition layer.
 5. The semiconductor structure ofclaim 1, wherein a composition of said gallium nitride layer at a bottomsurface thereof is different from a composition of said transition layerat a front surface thereof.
 6. The semiconductor structure of claim 1,wherein a gallium percentage in said gallium nitride layer at a bottomsurface thereof is different from a gallium percentage of saidtransition layer at a front surface thereof.
 7. The semiconductorstructure of claim 1, wherein a value of x+y of said transition layer ata front surface thereof is less than 0.3.
 8. The semiconductor structureof claim 1, wherein a thickness of said gallium nitride layer is greaterthan 0.1 microns.
 9. The semiconductor structure of claim 1, wherein athickness of said gallium nitride layer is greater than 2.0 microns. 10.The semiconductor structure of claim 1, wherein said transition layer issituated over a substrate.
 11. The semiconductor structure of claim 1,wherein said transition layer is situated over a silicon substrate. 12.The semiconductor structure of claim 1, wherein said transition layer issituated over an intermediate layer.
 13. The semiconductor structure ofclaim 1, wherein said transition layer is situated above an intermediatelayer comprising an alloy of gallium nitride consisting ofAl_(x)In_(y)Ga_((1-x-y))N.
 14. The semiconductor structure of claim 13,wherein a value of x+y of said intermediate layer is greater than 0.4.15. The semiconductor structure of claim 13, wherein a value of x+y ofsaid intermediate layer is different from a value of 1-x-y of saidtransition layer at a back surface thereof.
 16. The semiconductorstructure of claim 1, wherein said semiconductor structure is part of aFET.
 17. The semiconductor structure of claim 1, wherein saidsemiconductor structure is part of an LED.
 18. A method for forming asemiconductor structure, said method comprising: forming adiscontinuously graded transition layer comprising an alloy of galliumnitride consisting of Al_(x)In_(y)Ga_((1-x-y))N; forming a galliumnitride layer over said transition layer.
 19. The method of claim 18,wherein a value of y of said transition layer is selected from the groupconsisting of zero and substantially zero.
 20. The method of claim 18,wherein a value of 1-x-y at a front surface of said transition layer isgreater than a value of 1-x-y at a back surface of said transitionlayer.